Ion channels are cellular proteins that regulate the flow of ions, including potassium, calcium, chloride and sodium into and out of cells. Such channels are present in all animal and human cells and affect a variety of processes including neuronal transmission, muscle contraction and cellular secretion.
Humans have over 70 genes encoding potassium channel subtypes (Jentsch Nature Reviews Neuroscience 2000, 1, 21-30) with a great diversity with regard to both structure and function. Neuronal potassium channels, which are found in the brain, are primarily responsible for maintaining a negative resting membrane potential, as well as controlling membrane repolarisation following an action potential.
One subset of potassium channel genes is the KCNQ family. Mutations in four out of five KCNQ genes have been shown to underlie diseases including cardiac arrhythmias, deafness and epilepsy (Jentsch Nature Reviews Neuroscience 2000, 1, 21-30).
The KCNQ4 gene is thought to encode the molecular correlate of a potassium channel found in outer hair cells of the cochlea and in Type I hair cells of the vestibular apparatus, in which mutations can lead to a form of inherited deafness.
KCNQ1 (KvLQT1) is co-assembled with the product of the KCNE1 (minimal K(+)-channel protein) gene in the heart to form a cardiac-delayed rectifier-like K(+) current. Mutations in this channel can cause one form of inherited long QT syndrome type 1 (LQT1), as well as being associated with a form of deafness (Robbins Pharmacol Ther 2001, 90, 1-19).
The genes KCNQ2 and KCNQ3 were discovered in 1988 and appear to be mutated in an inherited form of epilepsy known as benign familial neonatal convulsions (Rogawski Trends in Neurosciences 2000, 23, 393-398). The proteins encoded by the KCNQ2 and KCNQ3 genes are localised in the pyramidal neurons of the human cortex and hippocampus, regions of the brain associated with seizure generation and propagation (Cooper et al. Proceedings National Academy of Science USA 2000, 97, 4914-4919).
KCNQ2 and KCNQ3 are two potassium channel subunits that form “M-currents” when expressed in vitro. The M-current is a non-inactivating potassium current found in many neuronal cell types. In each cell type it is dominant in controlling membrane excitability by being the only sustained current in the range of action potential initiation (Marrion Annual Review Physiology 1997, 59, 483-504). Modulation of the M-current has dramatic effects on neuronal excitability, for example activation of the current will reduce neuronal excitability. Openers of these KCNQ channels, or activators of the M-current, will reduce excessive neuronal activity and may thus be of use in the treatment of seizures and other diseases and disorders characterised by excessive neuronal activity, such as neuronal hyperexcitability including convulsive disorders, epilepsy and neuropathic pain.
Retigabine (D-23129; N-(2-amino-4-(4-fluorobenzylamino)-phenyl)carbamic acid ethyl ester) and analogues thereof are disclosed in EP554543. Retigabine is an anti-convulsive compound with a broad spectrum and potent anticonvulsant properties, both in vitro and in vivo. It is active after oral and intraperitoneal administration in rats and mice in a range of anticonvulsant tests including: electrically induced seizures, seizures induced chemically by pentylenetetrazole, picrotoxin and N-methyl-D-aspartate (NMDA) and in a genetic animal model, the DBA/2 mouse (Rostock et al. Epilepsy Research 1996, 23, 211-223). In addition, retigabine is active in the amygdala kindling model of complex partial seizures, further indicating that this compound has potential for anti-convulsive therapy. In clinical trials, retigabine has recently shown effectiveness in reducing the incidence of seizures in epileptic patients (Bialer et al. Epilepsy Research 2002, 51, 31-71).
Retigabine has been shown to activate a K(+) current in neuronal cells and the pharmacology of this induced current displays concordance with the published pharmacology of the M-channel, which recently was correlated to the KCNQ2/3 K(+) channel heteromultimer. This suggests that activation of KCNQ2/3 channels may be responsible for some of the anticonvulsant activity of this agent (Wickenden et al. Molecular Pharmacology 2000, 58, 591-600)—and that other agents working by the same mechanism may have similar uses.
KCNQ 2 and 3 channels have also been reported to be upregulated in models of neuropathic pain (Wickenden et al. Society for Neuroscience Abstracts 2002, 454.7), and potassium channel modulators have been hypothesised to be active in both neuropathic pain and epilepsy (Schroder et al. Neuropharmacology 2001, 40, 888-898).
Retigabine has also been shown to be beneficial in animal models of neuropathic pain (Blackburn-Munro and Jensen European Journal of Pharmacology 2003, 460, 109-116), and it is thus suggested that openers of KCNQ channels will be of use in treating pain disorders including neuropathic pain.
The localisation of KCNQ channel mRNA is reported in brain and other central nervous system areas associated with pain (Goldstein et al. Society for Neuroscience Abstracts 2003, 53.8).
In addition to a role in neuropathic pain, the expression of mRNA for KCNQ 2-5 in the trigeminal and dorsal root ganglia and in the trigeminal nucleus caudalis implies that openers of these channels may also affect the sensory processing of migraine pain (Goldstein et al. Society for Neuroscience Abstracts 2003, 53.8).
Recent reports demonstrate that mRNA for KCNQ 3 and 5, in addition to that for KCNQ2, are expressed in astrocytes and glial cells. Thus KCNQ 2, 3 and 5 channels may help modulate synaptic activity in the CNS and contribute to the neuroprotective effects of KCNQ channel openers (Noda et al., Society for Neuroscience Abstracts 2003, 53.9).
Retigabine and other KCNQ modulators may thus exhibit protection against the neurodegenerative aspects of epilepsy, as retigabine has been shown to prevent limbic neurodegeneration and the expression of markers of apoptosis following kainic acid-induced status epilepticus in the rat (Ebert et al. Epilepsia 2002, 43 Suppl 5, 86-95). This may have relevance for preventing the progression of epilepsy in patients, i.e. be anti-epileptogenic. Retigabine has also been shown to delay the progression of hippocampal kindling in the rat, a further model of epilepsy development (Tober et al. European Journal Of Pharmacology 1996, 303, 163-169).
It is thus suggested that these properties of retigabine and other KCNQ modulators may prevent neuronal damage induced by excessive neuronal activation, and such compounds may be of use in the treatment of neurodegenerative diseases, and be disease modifying (or antiepileptogenic) in patients with epilepsy.
Given that anticonvulsant compounds such as benzodiazepines and chlormethiazole are used clinically in the treatment of the ethanol withdrawal syndrome and that other anticonvulsant compounds e.g. gabapentin are very effective in animal models of this syndrome (Watson et al. Neuropharmacology 1997, 36, 1369-1375), other anticonvulsant compounds such as KCNQ openers are thus expected to be effective in this condition.
mRNA for KCNQ 2 and 3 subunits are found in brain regions associated with anxiety and emotional behaviours such as bipolar disorder e.g. hippocampus and amygdala (Saganich et al. Journal of Neuroscience 2001, 21, 4609-4624), and retigabine is reportedly active in some animal models of anxiety-like behaviour (Hartz et al. Journal of Psychopharmacology 2003, 17 suppl 3, A28, B16), and other clinically used anticonvulsant compounds are used in the treatment of bipolar disorder. Thus, KCNQ openers may be useful for the treatment of anxiety disorders and bipolar disorder.
WO 200196540 discloses the use of modulators of the M-current formed by expression of KCNQ2 and KCNQ3 genes for insomnia, while WO 2001092526 discloses that modulators of KCNQ5 can be utilized for the treatment of sleep disorders.
WO01/022953 describes the use of retigabine for prophylaxis and treatment of neuropathic pain such as allodynia, hyperalgesic pain, phantom pain, neuropathic pain related to diabetic neuropathy and neuropathic pain related to migraine.
WO02/049628 describes the use of retigabine for the treatment of anxiety disorders such as anxiety, generalized anxiety disorder, panic anxiety, obsessive compulsive disorder, social phobia, performance anxiety, post-traumatic stress disorder, acute stress reaction, adjustment disorders, hypochondriacal disorders, separation anxiety disorder, agoraphobia and specific phobias.
WO97/15300 describes the use of retigabine for the treatment of neurodegenerative disorders such as Alzheimer's disease; Huntington's chorea; sclerosis such as multiple sclerosis and amyotrophic lateral sclerosis; Creutzfeld-Jakob disease; Parkinson's disease; encephalopathies induced by AIDS or infection by rubella viruses, herpes viruses, borrelia and unknown pathogens; trauma-induced neurodegenerations; neuronal hyperexcitation states such as in medicament withdrawal or intoxication; and neurodegenerative diseases of the peripheral nervous system such as polyneuropathies and polyneuritides.
KCNQ channel openers have also been found to be effective in the treatment of stroke, therefore it can be expected that selective KCNQ openers are effective in the treatment of stroke (Schroder et al., Pflugers Arch., 2003; 446(5): 607-16; Cooper and Jan, Arch Neurol., 2003, 60(4):496-500; Jensen, CNS Drug Rev., 2002, 8(4):353-60).
KCNQ channels have been shown to be expressed in dopaminergic and cholinergic circuits in the brain that are associated with the brain's reward system, particularly the ventral tegmental area (Cooper et al., J Neurosci, 2001, 21, 9529-9540). Therefore, KCNQ channel openers are expected to be effective in hyperexcitability disorders that involve the brain's reward system such as cocaine abuse, nicotine withdrawal and ethanol withdrawal.
Potassium channels comprised of the KCNQ4 subunits are expressed in the inner ear (Kubisch et al., Cell., 1999 Feb. 5; 96(3):437-46) and opening of these channels is therefore expected to treat tinnitus.
Hence, there is a great desire for novel compounds which are potent openers of the KCNQ family of potassium channels.
Also desired are novel compounds with improved properties relative to known compounds, which are openers of the KCNQ family potassium channels, such as retigabine. Improvement of one or more of the following parameters is desired:
half-life, clearance, selectivity, interactions with other medications, bioavailability, potency, formulability, chemical stability, metabolic stability, membrane permeability, solubility and therapeutic index. The improvement of such parameters may lead to improvements such as:                an improved dosing regime by reducing the number of required doses a day,        ease of administration to patients on multiple medications,        reduced side effects,        enlarged therapeutic index,        improved tolerability or        improved compliance.        